PROJECT SUMMARY. Skin is the most susceptible organ to radiation damage. For example, ?-radiation, commonly used in radiotherapies, induces stromal expression of cytokines such as TGF?, a powerful chemoattractant that can stimulate skin metastasis. Unfortunately, the inability to treat radiation-induced phenotypic changes has made cutaneous metastasis a deadly phenomenon that is correlated with extremely poor quality of life in inflammatory breast cancer (IBC) and other lethal diseases. We seek to develop topical siRNA delivery agents that can penetrate skin and normalize radiation-triggered maladaptive signaling by releasing siRNA ?antidotes? when radiation is applied. These nanocarriers would represent a powerful platform technology with broad relevance in a wide array of skin-based radiation disorders. siRNA offers enormous potential as a therapeutic, yet its delivery to organs other than the liver poses a technological challenge. We propose to address this issue in skin by fusing two nascent delivery schemes with cutting-edge and complementary properties: (i) peptide and solution modifications that facilitate nanocarrier transit into dermal stroma; and (ii) stimulus-responsive polymers [mPEG-P(APNBMA)], designed in our labs, that encapsulate siRNA into nanocarriers with highly tunable properties and a versatile chemical transition that we previously explored using light. Light triggers cleavage of o-nitrobenzyl (o-NB) moieties in the P(APNBMA) side chains, leading to polymer charge reversal and stimulus-responsive gene silencing. We have compelling new evidence that the o-NB bonds in mPEG-P(APNBMA) also cleave in response to mild ?-radiation. Herein, we will exploit this surprising cleavage phenomenon in skin, to determine whether topical mPEG- P(APNBMA) nanocarriers release siRNA in response to ?-radiation and thereby halt IBC skin metastasis. Aim 1 will develop topical formulations of siRNA nanocarriers with both of the desired properties: transcutaneous penetration into dermis, and ?-radiation-triggered siRNA release in skin fibroblasts. It also will provide quantitative information regarding siRNA nanocarrier distribution and radiation-triggered gene silencing kinetics in skin. Aim 2 will evaluate two governing hypotheses in IBC metastasis: (i) Radiation-induced TGF? activation in skin fibroblasts triggers the epithelial-to-mesenchymal (EMT) transition in lymphatic IBC emboli, leading to IBC invasion and growth in skin; and (ii) On-demand suppression of TGF?, using radiation-triggered siRNA delivery, provides a potent and proportional strategy to eliminate cutaneous IBC dissemination. This aim will provide new fundamental information regarding radiation side effects and IBC dissemination, and it will establish preclinical siRNA dose regimens that will suppress radiation-induced metastasis in stroma. Our team combines synergistic expertise in nucleic acid delivery (Sullivan), polymer materials (Epps), metastatic cancer biology (van Golen), and breast cancer treatment (clinical consultant Cristofanilli). In the long-term, our work will foster new additive therapy platforms relevant to a spectrum of radiation-induced skin diseases